1. Field of Invention
The present invention relates generally to the field of nonaqueous electrolytes, electric current producing cells, and energy storage cells. More particularly, the invention pertains to nonaqueous electrolytes comprising (a) one or more solvents; (b) one or more ionic salts; and (c) one or more additives. Electric current producing cells, energy storage cells comprising non-aqueous electrolytes, and methods of making nonaqueous electrolytic solutions with aromatic phosphite compounds as stabilizers for halogenated salts in lithium and lithium ion rechargeable batteries, supercapacitors, and so on, are disclosed herein.
2. Description of Related Art
Electric current producing cells such as batteries consist of pairs of electrodes of opposite polarity separated by electrolytic solution, which includes a solvent and a solute. The charge flow between electrodes is maintained by an ionically conducting solute, i.e., a salt. The non-aqueous electrolytic solutions, which are used in lithium and lithium ion batteries, are made by dissolving lithium salts in a variety of organic solvents. In particular, nonaqueous electrolytes comprising lithium hexafluorophosphate (LiPF6) exhibit very good electrochemical stability and conductivity. However, LiPF6 is not thermally stable and readily decomposes by hydrolysis, as set forth in the following reactions:LiPF6→LiF+PF5  (1)LiPF6+H2O→2HF+LiF+POF3  (2)
Thermal decomposition of LiPF6 occurs at elevated temperatures (Reaction 1), and is accelerated in solution due to the reactions of phosphorus pentafluoride (PF5) and solvents because PF5 is not only a very strong Lewis acid that will catalyze the decomposition (or polymerization) of the electrolyte solvents but also a strong fluorinating agent that readily reacts with organic solvents. It is believed that PF5 is a major cause of thermal decomposition of the electrolytes of lithium ion batteries.
Hydrolysis of LiPF6 (Reaction 2) generally occurs due to the presence of protic impurities such as moisture, alcohols and acidic impurities in the electrolytic solution. Accordingly, water, alcohols and acidic impurities, especially hydrogen fluoride (HF) are undesirable in lithium and lithium-ion battery systems. The strong acid HF is especially harmful to batteries because it reacts with electrode active materials and corrodes the solid electrolyte interface (SEI), which results in poor battery performance.
It may appear that deactivation or removal of PF5 in electrolytic solutions will diminish or prevent the subsequent decomposition reactions of the electrolytes, and hence increase the thermal stability of the electrolytes, which in turn improves thermal stability and high temperature performance of batteries made therewith. This deactivation or removal of PF5 can be achieved by complexing PF5 with a Lewis base. Zhang et al. found that tris(2,2,2-trifluoroethyl)phosphite, which has a Lewis basic P(III) center, can stabilize an electrolyte of 1.2M LiPF6 in EC/PC/EMC (3:3:4) for two weeks at 60° C. and act as a flame-retardant co-solvent for nonflammable electrolytes in lithium ion batteries (see Electrochem. Solid-State Letters, 5 (9), A206-A208 (2002), and J. Power Sources, 113, 166-172 (20003), both of which are hereby incorporated by reference in their entirety). U.S. Pat. No. 6,939,647 (incorporated by reference in its entirety) discloses electrolytes containing 1-50 wt % of trialkyl phosphites such as partially fluorinated alkyl phosphites. Li et al. also reported the stabilization of 1.0M LiPF6 in EC/DMC/DEC (1:1:1) with Lewis bases such as pyridine, hexamethoxycyclotriphosphazene, and hexamethylphosphoramide by the formation of base:PF5 complexes (see J. Electrochem. Soc., 152 (7), A1361-A1365 (2005); incorporated herein by reference in its entirety).